Activated carbon has been used for removal of impurities and recovery of useful substances from liquids and gases because of its high adsorptive capacity. Generally, “activation” refers to any of the various processes by which the pore structure is enhanced. Typical commercial activated carbon products exhibit a surface area (as measured by nitrogen adsorption as used in the B.E.T. model) of at least 300 m2/g. Common carbon sources include as resin wastes, coal, coal coke, petroleum coke, lignite, polymeric materials, and lignocellulosic materials including pulp and paper, residues from pulp production, wood, nut shell, kernel, fruit pit, petroleum, carbohydrates, and bone. Typical activation processes involve treatment of carbon sources either thermally with an oxidizing gas or chemically often with phosphoric acid or metal salts such as zinc chloride. U.S. patent No. RE 31,093 teaches a chemical activation of wood-based carbon with phosphoric acid to improve the carbon's decolorizing and gas adsorbing abilities. U.S. Pat. No. 4,769,359 teaches a method of producing activated carbon by treating coal cokes and chars, brown coals or lignite with a mixture of NaOH and KOH and heating to at least 500° C. in and inert atmosphere.
The activated carbon could be in the form of granules, spheres, monoliths, beads, powders or fibers.
California has adopted Enhanced Vapor Recovery (EVR) regulations for the systems installed at gasoline dispensing facilities for controlling gasoline vapors emitted during the fueling of storage tanks (Phase I) and the refueling of vehicle fuel tanks (Phase II). The gasoline distribution facilities (herein “gas stations”) must comply with such adapted regulation by April 2009.
To comply with the new regulatory, about 80% of California gas stations have balanced EVR systems, and about 20% uses vacuum assisted systems. Balanced systems are inherently compatible with on-board refueling vapor recovery (ORVR) equipped vehicles and are generally less expensive than vacuum assisted systems; therefore, they are the major emission control systems adopted by the gas stations.
Vapor recovery systems include all associated dispensers, piping, nozzles, couplers, processing unit (also known as “processor”), underground storage tanks, and any other equipment or components necessary for the control of gasoline vapors during Phase I or Phase II refueling operations at the gas stations. The equipment used to control hydrocarbon emissions from gas station storage tanks is typically known as a storage tank vent “processor.” Methods that may be considered for controlling such hydrocarbon emissions include oxidizing the emitted hydrocarbons to carbon dioxide and water, or capturing the emitted hydrocarbons and returning them to the storage tank via a vapor recovery system.
FIG. 1 shows a simplified gas station fuel dispensing system equipped with a balanced vapor recovery system. It includes at least one storage tank 100, typically an underground storage tank or also known as “UST”, containing liquid fuel 110 and a vapor space 115 above the liquid fuel level that its volume is commonly referred to as “ullage.” The gas station may have one or more fuel dispensing units 201 and 202, typically known as “gas pumps”. In FIG. 1, a submersible pump 120 is shown on top of tank 100, although it may be located elsewhere. The submersible pump 120 may be a turbine pump or other types of pump known in the art. As is typical with pumping systems, submersible pump 120 may draw liquid 110 up from within tank 100, through an intake device 125 such as a filter or screen, through a pickup line 122, into submersible pump 120, and then out of the pump through pump outlet line 130 which feeds the fuel dispensing units 201, 202. For gas stations equipped with a balanced vapor recovery system, an air/vapor return line 140 is provided between fuel dispensing units 201, 202 and back to tank 100.
The pressure in tank 100 may vary; therefore, typically a pressure valve (PV) 150 is provided to regulate the tank pressure to values near atmospheric, for example not allowing the tank pressure to depart from atmospheric pressure by more than a few inches water column (w.c). Furthermore, the pressure valve 150 may maintain the tank pressure between an upper (typically slight positive) pressure and a lower (typically slight negative or vacuum) pressure. The pressure valve 150 may comprise one or more physical valves, with one or more piping lines, pressure measuring or detecting devices, control devices, etc. Moreover, several underground storage tanks 100 may be manifolded together to utilize a common pressure valve 150. A vent device 160 may be provided, which may comprise a cover to keep out rain and a flame arrestor for safety. Balanced systems function by operating as a closed system between pressures of about 1.5-3 inches w.c. down to negative pressures (e.g., vacuum) of 3-10 inches w.c.
FIG. 2 depicts a situation when fuel is pumped out of the underground storage tank 100 to a non-ORVR (onboard refueling vapor recovery) vehicle 301. In this example, a metered amount of gasoline from fuel dispensing unit 201 travels though dispensing hose 230 to nozzle 211, and from there into the gas tank of non-ORVR vehicle 301. Simultaneously, an approximately equal volume of air/gasoline vapor is returned through dispenser air/vapor return line 240 back to the fuel dispensing unit 201 and from there through dispenser air/vapor return line 140 to the storage tank 100. Much of this air/gasoline vapor is from within the gas tank of vehicle 301, from which it is displaced by the incoming gasoline. The dispensing hose 230 and dispenser air/vapor return line 240 may be a pair of hoses or a coaxial hose.
In FIG. 2, the approximately equal exchange of fuel from tank 100 to vehicle 301 and air/vapor return from vehicle 301 back to tank 100, results in little pressure change in storage tank 100.
FIG. 3 depicts a situation when fuel is pumped out of the storage tank 100 to an ORVR vehicle 302. The operation is similar to that described above, except that in an ORVR vehicle 302 most of the air/vapor displaced from the vehicle tank by the incoming fuel is vented through the vehicle's own emission control system, the gasoline vapors are adsorbed by the vehicle's activated carbon canister, and the purified air vents to the atmosphere. As denoted by 241, little air/vapor flows back through dispenser air/vapor return line 240, and only a reduced volume of air (and largely devoid of hydrocarbons) is returned to the storage tank 100. The reduced volume of air may be only about 10% of the liquid fuel flow that went into vehicle 302. A net pressure within the storage tank 100 reduces, thereby inducing a flow 161 of makeup air devoid of hydrocarbons (in an amount about 90% of the liquid flow) in through vent device 160 and through the vacuum relief function of pressure valve 150.
In this description, the pressure valve (PV) 150 is used to describe a valve or valves operable under certain conditions to prevent the storage tank 100 from experiencing either an over pressure situation or an under pressure (excessive vacuum) situation. Typically such a system of valves allows air or air/vapors to move in and out of the storage tank 100 under controlled pressure conditions.
Due to the fluid flows as described here, while the gas station is open for business and actively filling a proportion of ORVR-equipped vehicles with gasoline, the net pressure within the storage tank 100 remains under a slight vacuum. After a number of ORVR-equipped vehicles 302 have been filled, the storage tank 100 vacuum may drop low enough for the vacuum relief function of PV valve 150 to open and allow fresh air 161 to enter the storage tank 100 to make up for the dispensed liquid fuel. The makeup air 161 is devoid of hydrocarbons and dilutes the air in the tank ullage 115, thereby reducing the hydrocarbon concentration in the vapor phase. To restore equilibrium, the vapor pressure of the gasoline causes it to partition from the liquid fuel volume 110 to the ullage 115 vapor phase, resulting in a volume/pressure increase within the storage tank 100. While the gas station is open and actively fueling vehicles, the net result is such that the storage tank 100 pressure remains negative and the concentration of gasoline vapors in the tank ullage 115 remains below saturation (for example at about 85%-98% of saturation).
FIG. 4 depicts the situation during periods when the station refueling is inactivity, for example during slow business hours or at night. The partitioning of gasoline from the liquid 110 to the vapor phase 115 to reach a state of vapor-liquid equilibrium (VLE) can cause the storage tank pressure to increase to a level that exceeds the upper pressure limit of pressure valve(s) PV 150, which causes the valve to open and allow air/gasoline vapor 165 to vent to the open atmosphere. The new California Phase II regulations require that the 30-day-average pressure within a tank must not exceed 0.25 inches w.c. (in calculating this average, negative pressures are treated as a value of zero gauge pressure), and the daily pressure cannot exceed 1.5 inches w.c. One purpose of this pressure requirement is to minimize fugitive emissions that can leak from a storage tank and associated equipment. Additionally, vapor emissions may not exceed 0.38 lbs/1000 gallons of fuel dispensed, which implies a total vapor recovery rate of better than 95%.
The balanced EVR systems relies on a relatively “high” vacuum (down to about −8 inches w.c.) to minimize vapor venting emissions. The pressure valve required for regulating the vacuum or pressure within the tank renders the system troublesome. It must be certified and demand regular maintenance or replacement.
U.S. Pat. No. 5,305,807 discloses an auxiliary vapor recovery device for use with a fuel dispenser that includes a vacuum pump and a canister containing adsorbent for removal of hydrocarbons from a vapor/air mixture. The vacuum pump draws air/vapor from the ullage of storage tanks through the canister for removal of hydrocarbon vapor and release of air. Additionally, the vacuum pump draws desorbed gasoline vapors from the spent adsorbent canister. U.S. Pat. No. 6,478,849 teaches a vapor recovery system for fuel storage tank that includes a pair of adsorbent canisters for alternative recovering volatile organic compounds (VOC) from the fuel tank ullage. While one canister adsorbs VOC, the other canister is regenerated by exhausting the VOC vapors from the VOC canisters back into the fuel tank ullage using a vacuum pump. These aforementioned vapor recovery systems require vacuum pump to actively draw vapor/air in the ullage of the fuel tank and desorb the VOC to regenerate the spent adsorbent. The requirement of vacuum pump in such vapor recovery systems consumes additional energy for operation and increases burden in maintenance.
U.S. Pat. No. 6,763,856 teaches a method for controlling pressure in the ullage space of an underground storage tank of volatile liquid fuel. The vapor/air in the ullage is treated or conditioned inside a gaseous flow conditioning apparatus to increase the fuel vapor concentration of the gaseous flow toward saturation, and then released into the fuel tank. The gaseous flow conditioning apparatus may contain at least one chamber where the gaseous flow is passed through a liquid fuel mist chamber or a close proximity of a fuel-wetted mesh. Additionally, the gaseous flow conditioning apparatus may comprise a chamber in which the gaseous flow is entrained into a stream of liquid fuel, and then delivered into a volume of liquid fuel inside the fuel tank. This method of controlling vapor emission requires a separate gaseous flow conditioning apparatus that conditions the gaseous flow in a manner to enhance vapor-liquid equilibrium prior to delivering the gaseous flow into the fuel tank, either to the ullage portion or liquid-filled portion of the fuel tank. The separate gaseous flow conditioning apparatus requires additional capital and installation cost.
Accordingly, there is a need for vapor recovery systems that do not require the use of vacuum pump, or other forced air/vapor system, thereby allowing the systems to control the vapor emission passively, yet effectively at lower energy consumption and maintenance cost.
Furthermore, there is a demand for vapor recovery systems that do not require additional gaseous flow conditioning apparatus in order to reduce capital, operation, and maintenance costs.
It is, therefore, an object of the present to provide for vapor recovery systems that effectively control the vapor emission from the gas station, yet do not require a vacuum pump to actively draw vapor/air from the ullage space and/or a separate gaseous flow conditioning apparatus, thereby rendering the ease of operating and maintaining the systems, as well as lowering the energy and cost for operating such systems.
It is another object of the present invention to provide vapor recovery systems that meet the new California regulations for vapor emission of less than 0.38 lbs/1000 gallons fuel dispensed yet without pressure relieve valve in order to simplify system operation and maintenance, as well as to maintain a UST pressure near ambient and ensure regulatory pressure profile compliance of less than 0.25 inches w.c.
It is yet another object of the present invention to provide vapor recovery systems having means to enhance the gasoline VLE equilibrium in the tank ullage, thereby minimizing vapor growth and potential venting of air and hydrocarbons to the atmosphere.
It is a further object of the present invention to provide canisters containing regenerable adsorbents that are capable of controlling the vapor emission level to below 0.38 lbs/1000 gallons fuel dispensed (i.e, greater than 95% control efficiency).
Other objects, features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.